The present invention relates to electronic devices, and, more particularly, to semiconductor devices useful in controlling battery-backed circuitry.
Power failures for computer systems can lead to loss of all information held in volatile memory, and power surges can damage components. Thus power down and power up routines are typically built into computer systems, and sensors for power failures together with reset signal generators and battery backups are available. Known devices combine a power failure sensor and a battery switch that outputs a power fail signal when power supply voltage drops from about 5 volts down to about 3.75 volts and further switches to battery power when the power supply voltage drops to about 3 volts. The power fail signal can be used as a reset signal for microprocessors and microcontrollers and a battery backup can hold program and data in memory until the power supply is restored. See Lee and Dias, U.S. Pat. No. 4,730,121. Such devices can rely upon hystersis in the comparator to avoid erratic reset signals.
Such devices have a problem if the circuit for detecting the restoration of power supply voltage is based on a lithium battery and the battery is discharged. Indeed, a locked in power fail signal is possible on power up because the switches isolating the battery from the power supply are inoperative, and the rising power supply voltage can charge up a lithium battery to a level that indicates a power failure because a lithium battery discharges to an open circuit state rather than a short circuit state.
The present invention provides a battery discharge current for both fast rising and slow rising power supply voltages.